The good news is that you are talking with the instrument. The bad news is that you are getting a error from the instrument. The instrument is complaining about a settings conflict, meaning that you are trying to do something to a module that you can't do or something along those lines. I'm guessing that the 3721 doesn't play with the Switching commands. It is really just a MUX for the thermocoupler measurements.
Try finding a module that is actually meant for switching and see if you get your error then.
